I came across this sentence and I just wanted to confirm that "〜なさ" in this context is used a means to be assertive, more so than "〜ない".
文章 (Taken from a Peanuts book, this is Lucy speaking to Snoopy) 「ルーシー:あなたにちょっと提案があるんだけど... あなたの人生をもっと意味深くする提案よ... でも、興味なさそうね...」「スヌーピー:犬の人生に意味は要らないのさ...」
Where normally I am used to seeing a "興味ない" I see "興味なさ".
From what I know, the "さ" particle is used for assertion in sentence endings.